This is a very old
metal mask from the Newar people of the Kathmandu Valley.
Among the Newar we find both Hindu and Buddhists - both with
ancient and highly esoteric tantric traditions. The Newari
people are also well known for having an incredible number of
religious festivals during the course of the year. I'd go
so far as to venture a guess that there may be no other
community in the world with so many religious holidays. Its
truly incredible. Naturally masks play an important role
in a good number of these events, and this old mask is a product
of the Newar tantric dance tradition.

This particular
character is possibly "Lakhey" - the demon who became a member
of the Goddess Durga's retinue. However, if so, it lacks the
exaggerated canine teeth we expect to see in a Lakhey mask.
It may well be "Bakhu" the rakshasa - another demon
character from the dances, but here again it is impossible to
say with certainty - though the iconography makes it a likely
candidate.